Given these new examples of evolutionary change, it seems clear that human evolution has continued at the very least until the recent past, nor is there any reason to think it will ever cease. An obvious but far-reaching conclusion follows. Evolution and history are not two distinct processes, with one following another like the change between royal dynasties. Rather, evolution and history overlap, with the historical period being overlaid on a still continuing process of evolutionary change. The implications are clearly of possible interest to historians and social scientists. Historians are concerned with motivation, but seldom consider sexual selection as a driver of national politics. Students of the Mongol empire have proposed many sophisticated reasons for the expansion of the Mongol empire, such as Genghis's supposed desire to prevent any future group of steppe dwellers rising to power in the way the Mongols had done. The discovery that 8% of Asian men in the lands ruled by Genghis Khan carry the Y chromosome of the Mongol royal house offers a quite different motive, but one of unusual specificity.
Sexual selection, in this case the effort by one male to propagate his genes at the expense of others, has been a powerful force throughout primate history, from chimpanzee societies to those of the Yanomamo. It has operated with little change in more complex societies, especially during times when access to women was one of the accepted rewards of power. Even in many contemporary societies, where at least a pretense of monogamy is expected of rulers, the old instincts have not disappeared. True, procreation played no evident role in the drive to power of dictators like Hitler or Stalin. But Mao Tse-tung, as revealed in the memoir by his personal physician Li Zhisui, lived like an emperor, with villas and swimming pools and a stream of girls procured by the Cultural Work Troupe of the Central Garrison Corps. "He was happiest and most satisfied when he had several young women simultaneously sharing his bed," writes his unadmiring Boswell.344 Presidents John F. Kennedy and Bill Clinton conducted affairs while serving in the White House. "I don't know a single head of state who hasn't yielded to some kind of carnal temptation, small or large. That in itself is reason to govern," said François Mitterrand, the French president whose funeral in 1996 was attended by both his wife and his mistress.^ Yet the drive for reproductive success is not a motive cited in many histories. Perhaps the possibility that a brute desire to procreate might drive the affairs of state is a concept that historians find too gross to contemplate.
Given that physical characteristics, such as the ability to digest lactose, have evolved in recent history, so too may have many other traits, including changes in social behavior. At least two conditions are necessary for the human genome to be significantly modified: there must be a selective pressure applied steadily for several generations, and those who adapt to the pressure must have more descendants than others. Such conditions may have occurred quite often in the human past, although it is hard at present to identify them. Even evolutionary changes need not be permanent. The aggressiveness of the Yanomamo could have a lot to do with the marginal nature of the environment in which some of them live. Under conditions in which aggressive men have more children, genes that favor aggression would become more common. If the
Yanomamo should suddenly become peaceful traders for many generations, then a new set of genes might be favored. The fierce Vikings of the tenth century became the peaceful Scandinavians of today. A cultural explanation is usually taken for granted, on the assumption that genes cannot change so quickly. But maybe the speed with which natural selection can act in human populations has been underestimated. Biologists are only just beginning to understand the genes that affect social behavior, some 30 of which have so far been detected, mostly in various species of laboratory animal. One of the most interesting findings is of a genetic mechanism for bringing about quick evolutionary change in a gene for behavior.1 346 A possible subject of future inquiry is whether longstanding traits of certain societies may have an evolutionary basis, perhaps because over many generations they allowed people with a certain kind of personality to enjoy greater reproductive success than others. Some scholars have remarked on long term cultural differences between societies of East and West. Richard E. Nisbett, a social psychologist at the University of Michigan, believes there are "dramatic differences in the nature of Asian and European thought processes," principally that Westerners view the behavior of physical objects and organisms as being governed by precise rules, whereas East Asians seek to understand events in terms of the complex web of interrelationships in which they are embedded. The social structures of Europe and China are built to match, in Nisbett's view, with Asian societies being interdependent and Western
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